1. Field of the Invention
This invention relates generally to a polycarbonate resin with a reduced volatile chlorine content and a method of producing such a polycarbonate resin, and more particularly to a polycarbonate resin which, upon held at normal temperatures, volatilizes a reduced amount of a chlorinous matter and so has stable quality, and a process for producing such a polycarbonate resin.
2. Background Art
Polycarbonate resins are generally produced by a two-phase interface polycondensation process wherein an aqueous solution of an alkali metal salt of diphenol, e.g., bisphenol A is reacted with phosgene in the presence of an organic solvent, or by a melt polycondensation process wherein the same reaction takes place in the absence of an organic solvent. So far, many studies have been made of impurities contained in phosgene used as the starting material in these processes.
Among them there are techniques for reducing the amount of chlorine contained in phosgene from the order of several hundred ppm to the order of a few tens ppm.
For instance, U.S. Pat. Nos. 3,230,253 and 3,331,873 disclose a method of treating phosgene with phenols or activated carbon (active carbon) for adsorption and removal of chlorine contained as an impurity in the phosgene. However, these patents are silent as to the use of such a chlorine-reduced phosgene as a raw material for polycarbonate production.
On the other hand, Japanese Patent Laid-Open Publication Nos. 62-297320 and 62-297321 teach that phosgene contains as an impurity carbon tetrachloride having a higher boiling point than that of phosgene, and that when a polycarbonate resin is produced using phosgene as a raw material, the carbon tetrachloride remaining in the polycarbonate resin causes generation of hydrogen chloride during molding of the resin due to the heat applied. For this reason, these publications propose reducing the amount of carbon tetrachloride in phosgene to a certain low level.
The publications, however, teach nothing about the phenomenon of gradual volatilization of a chlorinous matter from the resin when it is allowed to stand at normal temperatures after molding.
Phosgene is generally produced by the reaction of CO with Cl.sub.2 using an activated carbon as a catalyst. This reaction arrives at equilibrium at its final stage. To reduce the amount of Cl.sub.2 to be contained in the resulting phosgene, therefore, the CO/Cl.sub.2 ratio is to be shifted in such a direction that CO becomes excessive. The use of a large excess of CO, however, leads not only to CO gas loss but also to an unacceptable increase in the amount of the phosgene inevitably discharged as an off-gas together with the CO gas, whereby the production cost is undesirably increased.
Accordingly, in the practical operation in most of phosgene plants, the excessive amount of CO is usually reduced to the limit at which phosgene loss is minimized. Consequently, phosgene products now available usually contain a certain level of Cl.sub.2.
It was discovered by the present inventors that the presence of chlorine in the starting phosgene for the production of a polycarbonate resin correlates with the gradual volatilization of a chlorinous matter from a molded product of the polycarbonate resin.
Thus, it has been found that the chlorine present in the starting phosgene causes some form of chlorination at a certain site of raw materials at an initial reaction stage in the preparation process of polycarbonate, and the thus formed chlorinated portion remains unchanged throughout the preparation process. The chlorinated portion still remains unchanged even during a molding process of the resulting polycarbonate; however, it may be gradually decomposed with time at normal temperatures to cause gradual volatilization of a chlorinous matter from the molded product (this a chlorinous matter can be detected as Cl ion).
On the other hand, when such carbon tetrachloride as mentioned above remains in polycabonate or when there are chloroformate group residues that are formed in the production process of a polycarbonate resin by the two-phase interface polycondensation technique, the generation of HCl is observed during the melt molding of the resin. However, these chlorinated groups must be clearly distinguished from the aforesaid chlorinated portion in starting materials produced by Cl.sub.2 in the starting phosgene.
In this regard, it is to be noted that the above chlorinated groups of very high reactivity change to HCl immediately upon the application thereto of heat on melt molding, and the HCl can be easily removed by washing the molded product with pure water.
In the case of the chlorinated portion in raw materials produced by Cl.sub.2 present in the starting phosgene, on the other hand, it can partly be decomposed due to the heat in the melt molding of a polycarbonate resin to generate and emit a chlorinous matter. However, most of the chlorinated portions remain unchanged through the molding. The remaining chlorinated portions are kept unchanged when the molded product is washed with pure water, but may gradually be decomposed by the action of light or the like to volatilize a chlorinous matter (detectable as Cl ion). Such chlorine is herein called "volatile chlorine".
The presence of such volatile chlorine has in fact been neglected, but now is found to cause various problems in connection with the quality or physical properties of a variety of recently developed molded articles of polycarbonate resins.
An object of the present invention is to provide a polycarbonate resin having a reduced volatile chlorine content, and a process of producing the same.